Bokashi fermentation of brewery's spent grains positively affects larval performance of the black soldier fly Hermetia illucens while reducing gaseous nitrogen losses

Marco Gebiola; Michael V Rodriguez; Alexandro Garcia; Andrea Garnica; Jeffery K Tomberlin; Francesca M Hopkins; Kerry E Mauck

doi:10.1016/j.wasman.2023.09.033

Bokashi fermentation of brewery's spent grains positively affects larval performance of the black soldier fly Hermetia illucens while reducing gaseous nitrogen losses

Waste Manag. 2023 Sep 30:171:411-420. doi: 10.1016/j.wasman.2023.09.033. Online ahead of print.

Authors

Marco Gebiola¹, Michael V Rodriguez², Alexandro Garcia³, Andrea Garnica³, Jeffery K Tomberlin⁴, Francesca M Hopkins⁵, Kerry E Mauck³

Affiliations

¹ Department of Entomology, University of California Riverside, Riverside, CA, USA. Electronic address: marco.gebiola@gmail.com.
² Department of Environmental Sciences, University of California Riverside, Riverside, CA, USA. Electronic address: mrodr149@ucr.edu.
³ Department of Entomology, University of California Riverside, Riverside, CA, USA.
⁴ Department of Entomology, Texas A&M University, College Station, TX, USA.
⁵ Department of Environmental Sciences, University of California Riverside, Riverside, CA, USA.

PMID: 37783136
DOI: 10.1016/j.wasman.2023.09.033

Abstract

Digestion of waste feedstocks by larvae of the black soldier fly Hermetia illucens (Diptera: Stratiomyidae) (BSF) results in proteins for animal feed and organic fertilizer with a reduced environmental footprint, but it can still have negative environmental effects through greenhouse gas (GHG) and ammonia (NH₃) emissions. Both biomass conversion by BSF larvae and associated GHG and NH₃ emissions can depend on substrate properties that may be optimized through microbial inoculation pre-treatments, such as bokashi fermentation. Here, we quantified the effects of bokashi fermentation of brewery's spent grains on BSF rearing metrics and associated GHG and NH₃ emissions at benchtop scale. We found that bokashi fermentation increased larval biomass by 40% and shortened development time by over two days on average, compared with unfermented spent grains. In line with increased larval growth, CO₂ emissions in BSF larvae treatments were 31.0 and 79.0% higher in the bokashi fermented spent grains and Gainesville substrates, respectively, compared to the unfermented spent grains. Adding BSF larvae to the spent grains increased cumulative N₂O emissions up to 64.0 mg N₂O kg substrate_dry^-1 but there were essentially no N₂O emissions when larvae were added to fermented spent grains. Bokashi fermentation also reduced NH₃ fluxes from the volatilization of substrate nitrogen in the BSF larvae treatment by 83.7-85.8% during days 7 and 9, possibly by increasing N assimilation by larvae or by reducing the transformation of substrate NH₄⁺ to NH₃. Therefore, bokashi fermentation may be applied to improve performance of BSF larvae on a common industrial waste stream and reduce associated emissions.

Keywords: BSF larvae; Circular economy; GHG emissions; N(2)O; NH(3).